1. Field of the Invention
The present invention relates to a solid-state imaging device and a manufacturing method thereof, and more specifically, relates to a so-called rear surface irradiation type soli-state imaging device, which allows incidence of light on a rear surface side being an opposite side from a surface formed with electrodes and wiring.
2. Description Related to the Prior Art
Conventionally, a CMOS imaging device is widely known as an imaging device to be mounted in a digital camera or the like. The conventional CMOS imaging device (hereinafter simply called imaging device) has a silicon substrate formed with photodiodes (hereinafter called PDs) on its surface, and electrodes, wiring, and the like provided on the silicon substrate to scan the photodiodes. Furthermore, a color filter, microlenses, and the like are formed on layers of the electrodes, the wiring, and the like, so light from an object passes through the microlens and the color filter and then is incident upon the PD through space between the electrode and the wiring and the like.
In the so-called front surface irradiation type imaging device as described above, it is known that the electrode, the wiring, and the like limit an aperture ratio. Especially, when increase in pixel number downsizes each individual pixel, it is known that layers of the electrode, the wiring, and the like have to be deep and the aperture ratio is further reduced.
Considering above, a rear surface irradiation type imaging device is known as an imaging device having such a structure that increase in a pixel number does not contribute reduction in an aperture ratio (U.S. Pat. No. 8,420,434 corresponding to Japanese Patent Laid-Open Publication No. 2005-150463). The rear surface irradiation type imaging device is an imaging device that is provided with electrodes, wiring, and the like on a back side of PDs when viewed from a light incident direction, so incident light passed through a microlens and a color filter reaches the PD without being blocked by the electrode, the wiring, and the like. Thus, the rear surface irradiation type imaging device facilitates increase in an aperture ratio, as compared with the front surface irradiation type imaging device.
Also, a widely available digital camera or the like of recent years has a focus detection function, which allows automatic focus adjustment. As a method of focus detection, there is known a so-called phase difference AF method in which two types of images having a horizontal (or vertical) phase difference are obtained and a focus position is calculated based on the distance between the images. For example, in a digital camera or the like that performs the phase difference AF, an imaging device is provided with a pair of pixels, composed of a first pixel having an asymmetric structure likely to receive light from a left direction with respect to an optical axis and a second pixel having an asymmetric structure likely to receive light from a right direction, as pixels (hereinafter called phase difference pixels) for use in the phase difference AF, and the focus position is calculated based on pixel values of these pixels.
In recent years, since the automatic focus adjustment is a standard function of a digital camera or the like, it is desired that the rear surface irradiation type imaging device is also provided with the phase difference pixels to allow performance of the phase difference AF. However, for example, the horizontally asymmetric structure of the phase difference pixel is formed by a light shielding film provided in front of the PD, and there is a problem that forming the asymmetric structure of the phase difference pixel using the light shielding film impairs performance originally expected of the rear surface irradiation type imaging device, i.e. increase in the aperture ratio.